In an aromatics complex, a variety of process units are used to convert naphtha or pyrolysis gasoline into benzene, toluene and mixed xylenes, which are basic petrochemical intermediates used for the production of various other chemical products. In order to maximize the production of benzene, toluene and mixed xylenes, the feed to an aromatics complex is generally limited from C6 up to C11 compounds. In most aromatics complexes, the mixed xylenes are processed within the complex to produce the particular isomer-para-xylene, which can be processed downstream to produce terephthalic acid. This terephthalic acid is used to make polyesters, such as polyethylene terephthalate. In order to increase the production of benzene and para-xylene, the toluene and C9 and C10 aromatics are processed within the complex through a toluene, C9, C10 transalkylation/toluene disproportionation (TA/TDP) process unit to produce benzene and xylenes. Any remaining toluene, C9, and C10 aromatics are recycled to extinction. Compounds heavier than C10 are generally not processed in the TA/TDP unit, as they tend to cause rapid deactivation of the catalysts used at the higher temperatures used in these units, often greater than 400° C.
When para-xylene is recovered from mixed xylenes by a selective adsorption process unit in the complex, the C8 feed to the selective adsorption unit is processed to eliminate olefins and alkenyl aromatics such as styrene in the feed. Olefinic material can react and occlude the pores of the zeolite adsorbent. The olefinic material is removed by passing a C8+ stream across a clay or acidic catalyst to react olefins and alkenyl aromatics with another (typically aromatic) molecule, forming heavier compounds (C16+). These heavier compounds are typically removed from the mixed xylenes by fractionation. The heavy compounds cannot be processed in the TA/TDP unit due to their tendency to deactivate the catalyst and are generally removed from the complex as lower value fuels blend stock.
Also during hydrocarbon processing, compounds composed of an aromatic ring with one or more coupled alkyl groups containing three or more carbon molecules per alkyl group may be formed. Formation of these compounds may be from processes used by petroleum refiners and petrochemical producers to produce aromatic compounds from non-aromatic hydrocarbons, such as catalytic reforming. As many of these heavy alkyl aromatic compounds fractionate with the fractions containing greater than 10 carbon atoms, they are not typically sent as feedstock to the transalkylation unit, and instead are sent to gasoline blending or used as fuel oil.